Jp8000 can do the duo timbral keyboard split, Gaia layers 3 voices on the one keyboard. (I don't think Gaia can split the keyboard... not 100% sure) This makes for a big difference in terms of playing. I think raw synthesis the Gaia wins. (triple tonal layering beats JP's 2)

I only "fooled around" (one night stand?) with the gaia in a guitar center and my response was meh? i'd definitely find a cheap jp-8080 (occasionally one slips through the cracks sub-$400) and then buy an sh-32 with the difference. I think you'd be really covered for roland va sound after that for equal $.

I know a guy who has a Gaia, the stuff he's done with it sounds great. I've mixed him live and it sounds really nice over a big PA, fat and juicy. If you go here: http://www.supermelody.com/ and click on the listen tab the track Tinder Hearted you can hear it. That's the arpeggiator on the Gaia playing the looped bass riff.

Anyway, both those synths are pretty good. The Gaia has some extra options like USB for better integration with a computer setup and more sounds courtesy of the sampled waveforms. If you're into either of those things or you just like buying things new so you get a warranty then they're good reasons to go with the Gaia. They're both pretty similar though so just go with whichever one draws you to it more.